Armature pose different in modes. Yes, I have reset the positions
It's really hard to see a "slight change" when all you provide is two pictures, one on top of the other.

I see you have two green bones, meaning they have constraints on them. May those be "Child of" or "Stretch to"constraints?
In which case, when you make a change in your rig, you need to go in each of these constraints and click either set inverse button for the "Child of", either Reset button for the "Stretch to".

If you don't wanna bother about going through each bone and clicking buttons everytime you change something, I have some python scripts for you.

EDIT: also check the IK's pole angles.